1. Field of the Invention
The present invention relates to an optical network. Particularly, the present invention relates to an optical network using a wavelength-division multiplexing transmission system or a time-division multiplexing transmission system.
2. Description of the Related Art
In order to improve the reliability of circuit service, a network has recently been proposed which is capable of healing a signal of a failure developed in an optical network. The circuit failure includes an unintentional cutoff of a transmission line, signal degradation, repeater""s trouble, etc. This type of optical network is normally designed so as to automatically detect the circuit failure and automatically perform switching between transmission lines. Such an optical network is called xe2x80x9csurvival networkxe2x80x9d.
There is also provided a survival network capable of performing switching to an SDH system or a SONET system corresponding to a synchronous network in particular to improve the healing capability of such a transmission network. The SDH system is an abbreviation of the Synchronous Digital Hierarchy System. Further, the SONET system is short for the Synchronous Optical Network.
As examples of its use, there are known (1) a 1:N type NPS (Nested Protection Switching) system which performs switching between a plurality of working lines and a plurality of protection lines, (2) a 4Fiber type BLSR (Bidirectional Line Switching Ring) system connected in a ring form by working lines and protection lines, etc.
The former example is described in, for example, Fiber Network Service Survivability, 1992 Artech House, INC,and T1X1.5/90-132. The latter example is described in Bellcore SONET BLSR Genertic Criteria GR-1230-CORE, 1993.
FIG. 2 is a diagram for describing the 1:N type NPS system. In FIG. 2, reference numerals 101 through 104 indicate transmission equipment respectively. Working lines 105 through 108 indicate bidirectional lines respectively. The bidirectional lines described herein are formed by two optical fibers. In FIG. 2, the bidirectional lines are shown by arrows indicated by bidirectional solid lines corresponding to respective one reference numerals.
The example shown in FIG. 2 has the following connections. The working line 105 is connected to terminating equipment 112 lying in the transmission equipment 101 and an add-drop multiplexer (branch-insertion equipment) lying in the transmission equipment 102. The line is connected to the terminating equipment or add-drop multiplexer by using an optical transmitter on the transmitting side and using an optical receiver on the receiving side. Further, the working line 105 is connected to the working line 106 through the add-drop multiplexer lying in the transmission equipment 102. The working line 107 is connected to the transmission equipment 102 and terminating equipment lying in the transmission equipment 104. The working line 107 is relayed by the transmission equipment 103. Reference numeral 113 indicates a repeater. Namely, the transmission equipment 103 does not have the ability to switch the bidirectional line 107 to a protection line. On the other hand, protection lines 109 through 111 are indicated by dotted lines in FIG. 2 respectively. The protection lines 109 through 111 connect all the transmission equipment 101 through 104 to one another through the use of the add-drop multiplexers within the respective transmission equipment. The respective transmission equipment have the ability to switch the working lines to the protection lines, respectively.
An example of switching to be done by the NPS system will next be described. A description will be made of how to perform switching when a failure occurs in the bidirectional line 107, for instance. Since the bidirectional line 107 is terminated by the transmission equipment 102 and the transmission equipment 104, these transmission equipment respectively have the ability to perform line switching. Therefore, when the failure occurs in the bidirectional line 107, the transmission of a signal is done using the protection line 110 and the protection line 111. The 1:N type NPS system shown in FIG. 2 lays or strings the working lines according to the capacity of a traffic (main signal) and can select the corresponding add-drop multiplexer or repeater within each transmission equipment.
FIG. 3 is a diagram for describing the 4Fiber type BLSR system. In FIG. 3, reference numerals 201 through 204 indicate transmission equipment respectively. Working lines 221 through 224 and protection lines 211 through 214 are respectively connected to one another in a ring form through add-drop multiplexers within the transmission equipment. The respective transmission equipment have the ability to switch the working lines to the protection lines, respectively.
A description will be made of a basic operation of the 4Fiber type BLSR system, which is related to line switching for a circuit failure. When a failure occurs in the working line 221 in FIG. 3, the transmission equipment 201 and 202 perform bidirectional line switching and thereby heals a signal through the use of the protection line 211. When failures occur in both the working line 221 and the protection line 211, i.e., failures such as a cable cut, etc. occur, a diverse route characterized by the ring type network is utilized. Namely, the 4Fiber type BLSR system performs line switching by means of the transmission equipment 201 and 202 to thereby allow the healing of a signal through the use of the protection lines 212 through 214. The method of healing the signal by the ring type in this way is characterized in that two clockwise and counterclockwise routes can be selected. The present 4Fiber BLSR system is provided by GR-1230-CORE.
The conventional survival network performs line switching to heal a traffic with a view toward healing failures such as a cable cut, cutting-off of an optical fiber and a breakdown in optical transmit-receive unit, etc.
However, the above-described 1:N NPS system needs an optical fiber according to the demand for traffics. However, when the number of optical fibers is insufficient, the system needs to additionally increase optical fibers between transmission equipment and thereby involves great installation cost.
The above-described BLSR system has the following drawbacks. Since the BLSR is of a 1:1 system, protection lines corresponding to transmission capacity of working lines must be installed. Accordingly, the capacity of the maximum traffic, which is necessary for an optical span, results in the total capacity of the ring. When only a traffic between the transmission equipment 201 and 202 takes the maximum capacitance value in the network in FIG. 3, for example, the entire ring must be set to the maximum capacitance value as well as to the maximum capacitance value of the working line 221. Namely, as the traffic concentrates on a given span in the ring, a problem occurs in use efficiency and economy.
Since the survival network needs the optical fibers corresponding to the transmission capacity due to fueling of an additional demand as described above, the cost of increasing the optical fibers and the efficiency of use of each optical fiber turn into problems.
The invention of the present application has been made to solve the above-described various problems.
A first object of the invention of the present application is to provide an optical network capable of flexibly selecting protection optical paths upon the occurrence of a failure without depending on the form of installation of an optical transmission line, e.g., an optical fiber.
A second object of the invention of the present application is to improve the efficiency of use of an optical transmission line, e.g., an optical fiber employed in an optical network.
A summary of a basic form of the invention of the present application will be briefly described. Summaries of respective various forms of the present invention disclosed in the specification of the present application will next be explained.
 less than Summary of Configuration of Optical Network According To the Invention of the Present Application greater than 
A typical optical network according to the invention of the present application is an optical network having at least a plurality of pieces of transmission equipment, and a plurality of optical paths connecting the plurality of pieces of transmission equipment and wherein wavelengths of predetermined transmission light are assigned to the optical paths to perform wavelength-division multiplexing digital transmission, and the plurality of optical paths include optical paths which connect the plurality of pieces of transmission equipment in a straight chain form.
More specific configurations of optical networks according to the inventions of the present application will further be illustrated by way of example as follows. These various inventions of the present application can provide so-called self-healing optical networks.
There is provided a wavelength division multiplexing transmission optical network having at least a plurality of pieces of transmission equipment, and a plurality of optical paths which connect the plurality of pieces of transmission equipment and wherein the plurality of optical paths are assigned wavelengths of predetermined transmission light and at least a transmission frame having overhead information or an overhead is used to perform digital transmission, and the plurality of optical paths include optical paths which connect the plurality of pieces of transmission equipment in a straight chain form.
Namely, the essential point of the wavelength division multiplexing transmission optical network resides in that the optical paths connected in the straight chain form are constructed so as to be available as protection optical paths. Thus, when a failure occurs in so-called mesh-like working optical paths, they can be healed with the protection optical paths. The optical network according to the invention of the present application can provide a so-called self-healing optical network.
The invention of the present application can be applied to both a wavelength division multiplexing transmission system and a time-division multiplex transmission system.
An optical path of light allocated according to the selection of a wavelength or time division, is called ┌optical path┘. Namely, a logical connecting path for a lightwave signal corresponds to the ┌optical path┘. On other hand, a physical connecting path constituting the optical path specifically is called ┌optical transmission line┘. Described specifically, for example, an optical fiber is a typical example. Accordingly, a plurality of optical paths can exist in one optical transmission line, for example. Namely, when the respective optical paths depend on the assignment of wavelengths, this communication system is normally called xe2x80x9cwavelength division multiplexind transmissionxe2x80x9d. When the respective optical paths are allocated according to the time division, this communication system is normally called xe2x80x9ctime division multiplexingxe2x80x9d.
Incidentally, the overhead held by the transmission frame indicates a region in which operation and maintenance information of the network are transferred. An automatic switching byte in the overhead is used to indicate both the transfer of a signal for controlling system switching between transmission terminals and alarm status with respect to breakdowns in a repeater and transmission equipment in a transmission system, and alarm status.
Switching control information about a failure, failure information of respective optical paths, wavelength address information, etc. lying in the automatic switching byte are needed for the purpose of the selection of the optical paths. While the transmission equipment are capable of performing digital transmission by at least using the transmission frame having the overhead, they have memory means which constitute wavelength address maps for storing therein at least optical multiplex information of the respective optical paths, e.g., the wavelength address information, and the failure information of the respective optical paths. In the case of the time division multiplex, the respective optical multiplex information result in time-division address information.
When a failure occurs in a working optical path, the switching control information about the failure and the wavelength address information are transferred or communicated between the transmission equipment based on information about the automatic switching byte in the overhead of the transmission frame, so that the corresponding optical path is switched over to another based on the switching information, the wavelength address information and the failure information. Incidentally, these specific examples will be described in the section of embodiments of the invention.
In the optical network, the optical paths can be divided into the working optical path and the protection optical path according to its logical connections. In the present optical network, however, the respective optical paths themselves can play both working and protection roles according to instructions from the respective transmission equipment. The working optical path is an optical path for transmitting a desired signal, whereas the protection optical path may be considered to be a name of an optical path used when some failure occurs in each optical path.
The invention of the present application is extremely useful for application to the typified SONET or SDH network known heretofore as the basic configuration of the optical network. In the SONET or SDH network, the byte for automatic switching is called xe2x80x9cAPS (Automatic Protection Switching) bytexe2x80x9d and normally comprises two regions of K1 and K2. The details of the APS byte will be described later.
Summaries of various forms of the invention of the present application, which are disclosed in the specification of the present application, will next be listed.
A basic idea of an optical network according to the present invention is that when a failure occurs in a working optical path, switching information including wavelength address information is communicated between transmission equipment and a signal is healed through the use of a protection optical path. However, the following forms are considered as optical networks for facilitating the forms thereof and performing switching at high speed and with ease. (1) The optical network has at least protection optical paths which connect the transmission equipment in a ring form. (2) The optical network has at least protection optical paths which connect the transmission equipment in a straight chain form. (3) The optical network includes at least two or more optical paths per optical transmission line. This form improves the efficiency of use of an optical fiber and enhances flexibility without depending on a physical form such as the optical fiber or the like. (4) As an optical network form of the present invention, the optical network of the present application may include at least two or more ring networks constructed by the working optical paths. (5) The basic operation of the wavelength division multiplexing optical network according to the invention of the present application is summarized as follows: The wavelength division multiplexing optical network according to the invention of the present application is a wavelength division multiplexing survival network having a plurality of pieces of transmission equipment, and optical paths which connect the plurality of pieces of transmission equipment and are assigned optical wavelengths and wherein at least a transmission frame having an overhead is used to perform digital transmission. Each of the transmission equipment has memory means constituting a wavelength address map for storing therein at least wavelength address a information of each optical path and failure information of each optical path. When a failure occurs in the working optical path, switching control information about the failure and wavelength address information are communicated between the transmission equipment based on an automatic protection switching byte lying in the overhead of the transmission frame, so that the faulty working optical path is switched over to the proper optical path based on the switching information, the wavelength address information and the failure information.
When the working optical paths and protection paths as optical paths are logically connected the transmission equipment to each other, the wavelength division multiplexing survival network can be constructed so that the working optical paths are respectively switched over to the protection optical paths based on the switching information, the wavelength address information and the failure information.
(6) There is provided the wavelength division multiplexing survival network as described in the items (1) through (5) wherein the above-described wavelength division multiplexing survival network includes at least two or more optical paths per optical transmission line.
(7) There is provided the wavelength division multiplexing survival network as described in the items (1) through (6) wherein the switching information includes at least wavelength addresses for working optical paths and switched states of transmission equipment.
(8) There is provided the wavelength division multiplexing survival network as described in the items (1) through (7) wherein the switching information includes at least wavelength addresses for working optical paths highest in importance and switching states of transmission equipment.
(9) There is provided the wavelength division multiplexing survival network as described in the items (1) through (7) wherein the switching information includes at least numbers of working optical paths highest in importance, numbers of transmission equipment which transmitted the switching information, and switched states of the transmission equipment.
(10) There is provided the wavelength division multiplexing survival network as described in the items (1) through (7) wherein the above-described wavelength division multiplexing survival network includes at least two or more ring networks constructed by the working optical paths.
(11) There is provided an optical network, comprising at least: a plurality of pieces of transmission equipment; and a plurality of optical paths which connect said plurality of pieces of transmission equipment to one another; wherein said optical paths are used as working optical paths or protection optical paths, and wavelengths of predetermined transmission light are assigned to said optical paths to perform wavelength division multiplexing digital transmission, and said plurality of optical paths have optical paths which connect said plurality of pieces of transmission equipment in a straight chain form.
(12) Furthermore, there is provided the optical network according to item (11), further including at least optical paths connecting said transmission equipment in a ring form and wherein said optical paths are capable of being used as protection optical paths.
(13) Furthermore there is provided the optical network according to item (11), further including at least optical paths connecting said transmission equipment in a straight chain form, and wherein said optical paths are capable of being used as protection optical paths.
(14) Furthermore there is provided the optical network according to item (12), wherein said optical paths connect a plurality of said transmission equipment in form of mesh-like and said optical paths are served as a working line.
(15) Furthermore there is provided the optical network according to item (13), wherein said optical paths connect plurality of said transmission equipments in form of mesh-like and said optical paths are served as a working line.
(16) There is provided a time-division multiplex transmission optical network, comprising at least: a plurality of pieces of transmission equipment; and a plurality of optical paths which connect said plurality of pieces of transmission equipment to one another, wherein said optical paths are used as working optical paths or protection optical paths, and said plurality of optical paths are assigned predetermined time division multiplex signals and at least a transmission frame having overhead information is used to perform digital transmission, and said plurality of optical paths include optical paths which connect said plurality of pieces of transmission equipment in a straight chain form.
(17) Furthermore there is provided the optical network according to item (16), further including at least optical paths connecting said plurality of pieces of transmission equipment in a ring form, and wherein said optical paths are capable of being used as protection optical paths.
(18) Furthermore there is provided the optical network according to item (16), further including at least optical paths connecting said plurality of pieces of transmission equipment in a straight chain form, and wherein said optical paths are capable of being used as protection optical paths.
(19) Furthermore there is provided the optical network according to item (17), wherein said optical paths connect plurality of said transmission equipments in form of mesh-like and said optical paths are served as a working line.
(20) Furthermore there is provided the optical network according to item (18), wherein said optical paths connect plurality of said transmission equipments in form of mesh-like and said optical paths are served as a working line.
The feature, the optical paths connect plurality of said transmission equipments in form of mesh-like and said optical paths are served as a working line, is more useful for many other modes of the present invention.
 less than Typical Example of Switching Determining Steps of Optical Network According to the Invention of the Present Application greater than 
A network for implementing communications in switching information in an optical network of the present invention is as follows:
There is provided a wavelength division multiplexing survival network having at least several pieces of transmission equipment, working optical paths which connect the transmission equipment to one another and are assigned optical wavelengths, and protection optical paths which connect the transmission equipment to one another and are assigned optical wavelengths and wherein each of the transmission equipment has memory means constituting a wavelength address map for storing therein at least information of wavelength addresses and failure information on the respective optical paths, and performs digital transmission by using at least a transmission frame having an overhead, which comprises the following switching determining steps shown by way of example.
(1) Step 1: a step for determining whether an automatic protection switching byte of the overhead shows a failure pattern.
(2) Step 2: a step for determining whether the automatic protection switching byte of the overhead is destined for the transmission equipment which received the same.
(3) Step 3: a step for starting a switching operation when the automatic protection switching byte is destined for the transmission equipment which received the same.
(4) Step 4: a step for transferring the automatic protection switching byte when the automatic protection switching byte is not destined for the transmission equipment which received the same.
 less than Summary of Transmission Equipment greater than 
The logical configuration of the optical network has centrally been described up to now. A summary of a specific physical configurational example of the logic configuration will next be explained. More specific and practical configurations of such equipment will be described in the section of the embodiments of the invention.
An example of an optical network according to the invention of the present application is shown in FIG. 5. It is needless to say that the invention of the present application is not limited to this example. The wavelength division multiplexing survival network comprises transmission equipment 11 through 14, optical fibers 15 through 18, optical path add-drop multiplexers 21 through 24, protection optical paths 31 through 34, and working optical paths 41, 42, 44-1 and 44-2. In FIG. 5, the optical paths show bidirectional optical paths. Since, however, the same optical fiber is used in the example, the bidirectional optical paths make use of different optical wavelengths.
Each of the transmission equipment is constructed so as to contain the following elements. Reference numeral 9 in FIG. 1 shows this example. It has at least (1) transmit-receive units 5 and 6 for the optical paths, (2) control means 3 for optical transmission, and (3) path switching means or units 4. Further, the control means 3 has a wavelength address map 2 used as memory means for storing wavelength address information about the optical paths and failure information about the optical paths therein, and overhead processing means or units 1. The wavelength division multiplexing survival network is connected in a ring form by the protection optical paths 31 through 34 through optical path add-drop functions. The working optical path 41 is terminated by the transmission equipment 11 and 12.
The respective transmission equipment 11 through 14 have optical transmit-receive units and add-drop functions for the optical paths, respectively. The respective transmission equipment have the ability to perform optical-path switching, based on the functions respectively. This switching is done by the path switching units 4.
The optical path add-drop multiplexer is optical equipment which is comprised principally of a wavelength-division multiplexer (WDM), an optical crossconnect, an optical repeater, an optical filter, an optical switch or optical circulator, etc. This equipment corresponds to a piece of equipment capable of selecting an arbitrary optical wavelength and providing add (Add: insertion), drop (Drop: branch) and through (Through: pass).